[ORC] Fix an iterator invalidation issue in JITDylib::defineMaterializing.

The loop body may add and remove entries in the symbol table so we can't hold
iterators to the entries. This commit updates the method to use the newly added
NonOwningSymbolStringPtr type as keys for removal instead.

Side note: This bug has been present since the introduction of the
defineMaterializing method, but the method is called rarely and DenseMap
resizes are also rare so we didn't see any fallout until a large program was
thrown at it. There's no testcase as I haven't been able to reproduce the
failure with smaller testcases.